1. Field of the Invention
The present invention relates to integrated circuit package technology. In particular, the present invention relates to resin-encapsulated integrated circuit packages.
2. Discussion of the Related Art
Conventional lead frames are typically formed on a metal strip which provides up to ten (10) units. A conventional lead frame includes a "die pad" for accommodating a semiconductor die, and inner leads and outer leads. A lead frame can be incorporated in a variety of integrated circuit packages, such as a quad flat pack (QFP) package and its many variations. In a QFP package, each bond pad provided on the semiconductor die is wire-bonded to an inner lead which, in turn, is electrically coupled to an outer lead. The inner leads are typically provided mold-locking features to allow proper positioning of the lead frame during the molding step which provides a plastic or resin encapsulation of the package. After encapsulation, the outer leads are trimmed and bent using custom trim and form tools to complete the electrical terminals or "leads" used for mounting the package on to a printed circuit board. Precise forming of the leads is necessary to ensure satisfactory board yield. Malformed leads can result in open or shorted solder joints because of aplanarity or skewed leads. In addition, even without such malformed leads, board yield in QFP packages is also diminished by open solder joints resulting from solder wicking up the leads.
The size of a prior art QFP package is limited by the dimensions of the semiconductor die plus about 3 mm on each side. For example, a 7 mm.times.7 mm QFP package can accommodate up to a 4 mm.times.4 mm semiconductor die. Clearance requirements on a printed circuit board can add another 2 mm on each side to the final foot print. Thus, a 7 mm.times.7 mm QFP typically has a footprint of 9 mm.times.9 mm, thereby providing an effective board density of approximately 20%.
Conventional QFP type packages are encapsulated in resin both at the top and the bottom of the semiconductor die. Consequently, conventional QFP packages cannot be made thinner than 1.4 mm. In addition, external lead "stand-off" requirements add to the height of the final printed circuit board assembly.
One important quality measure for an integrated circuit package is reliability. In a QFP package, a significant failure mode is the delamination of the mold compound from the back of a die pad. Delamination introduces moisture into the package and causes moisture-related failures.
One performance measure in a conventional QFP or any plastic package is thermal performance. Such a package is limited in its thermal performance because of a lack of a thermally conductive path to dissipate heat from the semiconductor die to the exterior. In many applications, a heat sink is included in the package. However, including a heat sink increases the material cost of such a package. Further, even if a heat sink is included, there are still typically multiple layers of epoxy through which heat must flow from the semiconductor die to the exterior.
A conventional QFP package is typically manufactured in an assembly process which requires a custom mold, a custom trim tool and a custom form tool. Thus, the tooling cost for manufacturing a new QFP package is high. For a given integrated circuit, rather than providing a package that is optimized specifically for its size and its number of input/output (I/O) terminals, a designer typically selects a package by matching the size and I/O terminals requirements of his integrated circuit as closely as possible to one of a few available QFP packages for which the tooling investment is already made. Clearly, the resulting QFP package is optimized for neither density nor material cost.
What is desired is a low cost, high density, high reliability integrated circuit package with flexible configuration.